ATLANTA – Georgia on Tuesday executed a man who killed a fellow inmate despite arguments from his lawyers that his execution was prohibited by the Constitution because he was intellectually disabled.

Warren Lee Hill was put to death by an injection of pentobarbital at the state prison in Jackson. The 54-year-old was pronounced dead at 7:55 p.m. He declined to make a final statement but did accept an offer to have a prayer read over him by a clergy member.

After reading the execution order, the warden left the room at 7:42 p.m. Records from previous executions show that the drug generally begins to flow within a minute or two of the warden leaving the room. Hill kept his head raised, looking out at the witnesses, for a couple of minutes and then laid back and took a few deep breaths before becoming still.

"Today, the Court has unconscionably allowed a grotesque miscarriage of justice to occur in Georgia," Brian Kammer, a lawyer for Hill, said in an emailed statement. "Georgia has been allowed to execute an unquestionably intellectually disabled man, Warren Hill, in direct contravention of the Court's clear precedent prohibiting such cruelty."

Hill was sentenced to serve life in prison for the 1986 killing of his 18-year-old girlfriend, who was shot 11 times. While serving that sentence, he beat a fellow inmate, Joseph Handspike, to death using a nail-studded board. A jury in 1991 convicted Hill of murder and sentenced him to death.

Hill was previously set to die in July 2012, February 2013 and July 2013, but courts stepped in at the last minute with temporary stays so they would have time to consider challenges filed by Hill's lawyers. State and federal courts rejected his lawyers' filings this time around, and the U.S. Supreme Court declined his request for a stay of execution at about 6:30 p.m. Tuesday.

Lawyers for Hill have long argued he is intellectually disabled and, therefore, shouldn't be executed. State law and a 2002 U.S. Supreme Court decision both prohibit the execution of the intellectually disabled.

But Georgia has the toughest-in-the nation standard for proving intellectual disability. It requires capital defendants to prove beyond a reasonable doubt that they are intellectually disabled in order to avoid execution on those grounds. The state has consistently said Hill's lawyers failed to meet that burden of proof.

Hill's lawyers have argued that Georgia's standard is unconstitutional because mental diagnoses are subject to a degree of uncertainty that is virtually impossible to overcome. But the standard has repeatedly been upheld by state and federal courts.

Hill's lawyers had asked the U.S. Supreme Court to consider Hill's case in light of a ruling it issued in May that knocked down a Florida law. The high court said in that ruling that defendants should have a fair opportunity to show the Constitution prohibits their execution. Hill's lawyers argued that ruling should also invalidate Georgia's tough burden of proof.

Days before Hill was to be executed in February 2013, his lawyers submitted new statements from the three doctors who had examined Hill in 2000 and testified at his trial that he was not intellectually disabled. In their new statements, the doctors wrote that they had been rushed at the time of Hill's trial, and new scientific developments had surfaced since then. All three reviewed facts and documents in the case and wrote that they believed Hill is intellectually disabled.

The State Board of Pardons and Paroles, the only entity authorized to commute a death sentence to life in prison, on Tuesday rejected Hill's clemency petition.